As a type of fuel cell, a polymer-electrolyte fuel cell (PEFC) is known. Compared with other types of fuel cells, a PEFC has a lower operating temperature (about −30 to 120° C.) and potential for reductions in cost and size, and therefore is expected to be a power source for automobiles, and others.
As shown in FIG. 5, a PEFC 1 is configured such that a membrane electrode assembly (MEA) 2, which is a principal component, is sandwiched by an anode-side separator 20, which includes a fuel (hydrogen) gas channel 21, and a cathode-side separator 30, which includes an air (oxygen) channel 31, to form a fuel cell 1 called as a single cell. The membrane electrode assembly 2 has a structure in which an anode-side electrode 15a, which is made up of a catalyst layer 13a and a gas diffusion layer 14a of anode side, is laminated on one side of a polymer electrolyte membrane 10 which is an ion exchange membrane, and on the other side thereof, a cathode-side electrode 15b, which is made up of a catalyst layer 13b and a gas diffusion layer 14b of cathode side, is laminated.
In a PEFC, a thin membrane of perfluorosulfonic acid polymer (Nafion® membrane, DuPont, USA), which is a fluorine-based electrolyte resin (ion exchange resin), is mainly used as the electrolyte membrane (see Patent Document 1 etc.) Moreover, since a thin membrane made up of an electrolyte resin alone does not provide enough strength, a reinforcing-membrane-type electrolyte membrane is used, in which an expanded porous reinforcing membrane (for example, a thin membrane formed by expanded PTFE or polyolefin resin, etc.) is impregnated with an electrolyte resin dissolved in a solvent and dried (Patent Document 2 and Patent Document 3, etc.)
Moreover, a fluorine-based electrolyte resin used in a PEFC exhibits a proton conductivity when it contains water. The proton conductivity varies depending on the amount of water content such that the lower the water content, the lower the proton conductivity. On the other hand, in fuel cells of recent years, for the purpose of system simplification and cost reduction, there is growing need for the operation at lower humidification levels; however, an operation at a low humidification level will lead to a decline of proton conductivity, which will result in a significant reduction in the power generation performance compared with at a high humidification level.
That is, in an operation at a low humidification level, an electroendosmosis occurs in which water moves toward the cathode side along with the movement of protons, and the anode side of the fuel cell gets drier. When the anode side becomes dry, the proton conductivity of the electrolyte of the anode side decreases and also the resistance of the entire cell increases, leading to a significant decline in the cell performance. To prevent such drying of anode side, attempts have been made to utilize a water back-diffusion effect, in which the product water generated by the proton oxidation reaction at the cathode side is effectively moved to the anode side, thereby preventing a performance decline in a low humidification state.
In general, decreasing the film thickness of the electrolyte membrane will reduce the moving distance of water back-diffusion, and also increase the concentration gradient of water between the dry anode side and the wet cathode side, thus making it possible to increase the mobility of water so that the product water at the cathode side moves to the anode side as water back-diffusion.                Patent Document 1: JP Patent Publication (Kokai) No. 2001-35510A        Patent Document 2: JP Patent Publication (Kokai) No. 2005-302526A        Patent Document 3: JP Patent Publication (Kokai) No. 2006-202532A        